Capturing the outcomes of assistive technology (AT) is essential to know what works and what is needed. People across the globe are working to close the gap of unmet need for AT, and to enable access to quality AT products and services. Inconsistent terminology, measures and a lack of coordinated systems prevent the sharing of knowledge, and the lack of articulated global priorities limit the usefulness of data for policymakers and resource allocation decisions. This paper provides methods of a global "Grand Challenge" project, run by the Global Alliance of Assistive Technology Organizations, and describes the resulting challenges. A multiphase, mixed method approach founded on participatory principles sought to capture multi-country and multi-stakeholder perspectives "from the ground up." A collaborative consensus method was adapted to document and distil AT challenge data from global regions, to refine this data with global stakeholder organizations, and to publish a shortlist of AT Grand Challenges. The 2022 Grand Challenges in Outcomes and Impact are 1) measuring need, 2) documenting inputs, 3) measuring outcomes, 4) measure impacts, 5) sharing data, and 6) informing policy. The challenge now remains to work together to resolve them.